This invention relates to geophysical exploration and more particularly to marine magneto-telluric geophysical exploration.
It has long been known that telluric currents circulate beneath the surface of the earth. The prior art suggests that the measurement of these naturally occurring currents will indicate the resistivity of the earth and hence the geophysical characteristics of the subsurface. However, these telluric currents are subject to variations caused by external influences, such as sunspot activity. Because these variations are unpredictable, measurement of telluric currents was little used as a geophysical exploration tool until Louis Cagniard devised a technique which accounted for these variations. His work is represented, for example, in U.S. Pat. No. 2,677,801, wherein he proposes measuring not only the telluric currents but also the magnetic field at a measurement station on the earth's surface. Cagniard teaches the relationship between the measured orthogonal components of the earth's magnetic field and the measured orthogonal components of the earth's telluric electric field.
The teachings of Cagniard have been successfully implemented in magneto-telluric geophysical exploration on land. In magneto-telluric exploration on land, the magnetic field sensors and the telluric current electrodes are precisely laid out with surveying instruments. In this manner, the measured orthogonal components of the magnetic and telluric electric fields are precisely known with respect to specified orthogonal geographic directions, which are usually north-south and east-west.
Cagniard also suggests, in FIGS. 5 and 6 of his U.S. Pat. No. 2,677,801 that marine magneto-telluric exploration can be carried out. Attempts to carry out marine magneto-telluric exploration have not been completely successful. It has not been possible to precisely align the underwater sensors with respect to the specified orthogonal directions. It is virtually impossible for a diver to use surveying instruments under water in order to align the sensors in the same way that alignment is carried out in making magneto-telluric measurements on the surface of the earth.
In accordance with the present invention, the requirements for precisely aligning magneto-telluric sensors with respect to specified geographical directions is obviated.